1. Field of the Invention
This invention relates to a gaseous fuel engine in which combustible gas such as hydrogen, methane, ethane or the like is used as fuel.
2. Description of the Prior Art
Recently, there have been proposed various gaseous fuel engines in which combustible gas such as hydrogen, methane, ethane or the like is used as fuel. Especially, a hydrogen engine is highly expected as a pollution-free engine since hydrogen forms neither carbon dioxide or poisonous unburned components.
However, if gaseous fuel is supplied to a conventional gasoline engine instead of gasoline, various problems will arise because of differences in physical and chemical properties between gasoline and the gaseous fuel.
The air-fuel ratio in a combustible mixture of gaseous fuel is incomparably larger than that of vaporized fuel like gasoline, and accordingly, if gaseous fuel is introduced into the cylinders from an intake passage in the form an air-fuel mixture, the amount of intake air reduces, which results in reduction of the engine output power. Especially, in the case of hydrogen, high burning speed of hydrogen is apt to cause backfiring in the intake passage or premature ignition of fuel.
In order to overcome the aforesaid problems, it has been, proposed to provide a gaseous fuel port with a gaseous fuel supply valve separately from the intake port and to introduce gaseous fuel through the gaseous fuel port separately from intake air. For example, in the gaseous fuel engine disclosed in Japanese Patent Publication No. 58(1983)-12458, the gaseous fuel supply valve is opened after closure of the intake valve, whereby the air charging efficiency is improved and at the same time, backfiring is prevented. In the gaseous fuel engine disclosed in Japanese Patent Publication No. 1(1989)-23659, the gaseous fuel supply valve is large in diameter and relatively small in lift and is opened near the BDC just before the end of the intake stroke, whereby the air charging efficiency is improved and at the same time, backfiring is prevented. Further, in the gaseous fuel engine disclosed in Japanese Patent Publication No. 59(1984)-14611, the gaseous fuel port is disposed diametrically opposed to the exhaust valve and intake ports are provided at 90.degree. from the gaseous fuel port and the exhaust valve so that arrival of the gaseous fuel to the exhaust port is delayed, thereby preventing premature ignition of the gaseous fuel.
However, when the gaseous fuel port is opened and the gaseous fuel is injected into the cylinder for a predetermined time just before or after closure of the intake valve, injection of fuel into the cylinder is effected at the start of the compression stroke after all and it becomes harder for the gaseous fuel to flow into the cylinder as the volume in the cylinder becomes smaller and the pressure in the cylinder increases. This result in a poor charging efficiency of the gaseous fuel. This problem is especially serious when the gaseous fuel is supplied under a relatively low pressure like in the case where hydrogen stored in hydrogen storage alloy is used as the gaseous fuel. If the pressure in the cylinder becomes higher than the gaseous fuel injecting pressure while the gaseous fuel supply valve is open, the air-fuel mixture can reverse into the gaseous fuel supply pipe and backfiring can occur.
Accordingly, it is preferred that the gaseous fuel port be as large as possible in opening area so that a required amount of gaseous fuel can be introduced into the cylinder in a short time. However when a poppet valve, which is normally used in a four-cycle reciprocating engine, is used to close and open the gaseous fuel port, the effective opening area inherently changes with the amount of lift of the valve and the time that the valve is full opened occupies only a small part of the valve opening time. This together with limitations by layout and/or arrangement of the camshaft makes it very difficult to satisfy both the requirements on the valve opening time and the opening area.
When the air-fuel mixture is made lean in order to prevent premature ignition, the amount of fuel to be fed to the cylinder is limited and accordingly, the engine output cannot be sufficiently increased.